How does minoxidil affect male fertility?

Doesn't. Topical Minoxidil probably has no effect on male fertility. Finasteride (propecia) has been implicated in reduced fertility that is reversible upon discontinuing the medicine.
It shouldn't. Minoxidil is an anti-androgen (or anti-testosterone) molecule. However, since it is applied on the scalp for hair loss, it really doesn't affect other parts of the body, including male fertility.
It does not. U may be thinking of the other fda approved product for hair loss called propecia (finasteride). There may be some potential with that drug.